The present invention relates to a fiber optic star coupler, and more particularly to a star coupler of the type having at least one input optical fiber, a plurality of output optical fibers, and a coupling region at which the input and output fibers are fused together.
Such star couplers are required, for example, for fiber optic distribution networks and for optical data bus systems. The operation of such a star coupler will be described with reference to FIG. 1, where input optical fibers 1 to 10 and output optical fibers 11 to 20 are connected at an optical coupling region 21. Assume, for example, that light is coupled into input optical fibers 1 to 10, with P.sub.1 to P.sub.10 being the associated input light energy. Coupling region 21 mixes light energies P.sub.1 to P.sub.10 in such a manner that, for example, the same percentage of light energy P.sub.1 to P.sub.10, if possible, exits from each of output optical fibers 11 to 20. That is, each one of the output optical fibers 11 to 20 individually carries the output light energy P.sub.1 /10+P.sub.2 /10+ . . . -P.sub.10 /10.
A publication by M. D. Bailey, entitled "Bitaper Star Couplers With Up To 1000 Fibre Channels," in Electronics letters, July 5, 1979, Volume 15, No. 14, pages 432 to 433, discloses the twisting together of a plurality (up to 100) of multimode, gradient profile optical fibers. Thereafter, the twisted location is heated in such a manner that the optical fibers are fused together. Additionally, a mechanical tensile stress is produced in the longitudinal direction of the optical fibers so that the cross sections of the individual optical fibers are reduced in the zone of the coupling region (the twisted and fused location). Such a coupling region is therefore configured as a full rod and, due to its internal light distribution, is therefore not suitable for use with monomode optical fibers.
Star couplers for monomode optical fibers are presently of interest from an economic point of view, for example for use with digital, high resolution television distribution networks. The reason for this economic interest is that monomode optical fibers have a significantly greater transmission bandwidth than multimode optical fibers and therefore permit a significantly greater data transmission rate, for example several Gbit/sec. Furthermore monomode optical fibers must be employed in optical transmission systems which operate with coherent light.
At present, star couplers for monomode optical fibers can be produced by fiber optic linkages of so-called 2.times.2 fiber optic taper couplers. Such a 2.times.2 taper coupler (FIG. 2) is composed of two input optical fibers, with the light energy from the input optical fibers being transmitted essentially uniformly to the output optical fibers. This is realized by fusing together the input and output optical fibers at coupling region 21'. Simultaneously, the cross-sectional areas of the individual optical fibers are reduced (tapered). At present, it is possible to assemble a monomode star coupler of several such 2.times.2 taper couplers.
FIG. 3 shows an example of the structure of such a monomode star coupler having four monomode input optical fibers to provide four input channels 101 to 104 and having four monomode output optical fibers to provide four output channels 110 to 140. Initially, optical input channel 101 is mixed with channel 102 and channel 103 is mixed with channel 104. This results in intermediate channels 101' to 104'. Then intermediate channel 101' is mixed with 103' and intermediate channel 102' is mixed with 104', resulting in output channels 110 to 140. A larger number of input and output channels requires a substantial number of 2.times.2 taper couplers, which is a considerable disadvantage. For Z=2.sup.N channels, the number of 2.times.2 taper couplers required is given by N.multidot.2.sup.N-1.
A few examples: for 8=2.sup.3 channels, twelve 2.times.2 taper couplers are required; for 32=2.sup.5 channels, eighty 2.times.2 taper couplers are required.
These 2.times.2 taper couplers must then be connected together in a defined manner and with the lowest optical losses, e.g. with the aid of splices produced by fusing together the optical fibers in an electric arc. Such a star coupler is therefore very cost-inefficient to manufacture, which is a drawback, and is additionally encumbered with high, annoying transmission losses.